(a) Field of the Invention
The present invention relates to an objective lens system for endoscopes having favorably corrected aberrations, especially distortion.
(b) Description of the Prior Art
As one of the conventional objective lens systems for endoscopes, there is known the retrofocus type of lens system illustrated in FIG. 1.
The retrofocus type objective lens system for endoscopes comprises a lens unit L.sub.1 having negative refractive power on the object side of an aperture stop S and a lens unit L.sub.2 having total positive refractive power on the image side of the aperture stop. The lens unit L.sub.1 has the negative refractive power for widening field angle of the objective lens system and the lens unit L.sub.2 has an imaging function, thereby composing a telecentric optical system having the characteristics of the objective lens system for endoscopes to have a large visual field angle and to allow the principal ray P to be perpendicularly incident on the image surface regardless of image height.
The reason for the objective lens system for endoscopes to have the telecentric characteristics is that the angle of incidence is limited for the rays capable of falling on the end surface of an image guide composed of an optical fiber bundle, a solid-state image pickup device such as CCD or a relay lens system having entrance pupil thereof located at infinite distance when such a member is arranged on the image surface of the objective lens system. In other words, transmission efficiency is lowered and images are darkened when the principal ray emerging from the objective lens system falls obliquely on the image surface.
Further, the objective lens system for endoscopes having the above-described composition can easily be assembled and manufactured at a low cost though the lens system has a small diameter of several millimeter since the outside diameters of the lens units L.sub.1 and L.sub.2 are nearly equal to that of the image guide though the lens system has a large field angle, and the lens system is compact and comprises a small number of lens elements.
Illustrated in FIG. 2 is another conventional example of objective lens system for endoscopes disclosed by Japanese Unexamined Published Patent Application No. 226315/59.
This retrofocus type objective lens system for endoscopes comprises a lens unit L.sub.1 ' having negative refractive power on the object side of a pupil position S and a lens unit L.sub.2 ' on the image side of the pupil position S, and has a composition similar to that of the objective lens system shown in FIG. 1. Though the pupil is located with the lens unit L.sub.2 ', no inconvenience is caused by making discussion on an assumption that the lens unit L.sub.2 ' is located as a whole on the image side of the pupil S since the lens unit L.sub.2 " arranged on the extremely object side in the lens unit L.sub.2 ' is composed of a plano-convex lens component having a plane surface on the side of incidence. However, the objective lens system shown in FIG. 2 uses, in the vicinity of the pupil S, a bar-shaped lens component L.sub.2 " which is longer in the direction along the optical axis than the diameter thereof.
Endoscopes are classified into flexible endoscopes which can be flexed freely and non-flexible endoscopes which cannot be flexed. The flexible endoscopes permit freely changing direction of visual field for observation, whereas the non-flexible endoscopes allows observation only in a certain definite direction and the forward viewing objective lens system shown in FIG. 1 permits observing objects located only in the front direction. Therefore, side viewing, oblique viewing and backward viewing objective lens systems are separately necessary to observe objects located in different directions through the non-flexible endoscopes.
In contrast, the objective lens system shown in FIG. 2 enables to observe objects located in different directions by using, in the bar-shaped lens component L.sub.2 ", various types of field direction changing prisms such as shown in FIG. 3A, FIG. 3B and FIG. 3C.
In the objective lens system shown in FIG. 2, it is practically difficult to arrange an aperture stop having a light-shielding function at the pupil position S since it is to be located in the lens component L.sub.2 '.
FIG. 4 shows an optical system which is composed of the objective lens system shown in FIG. 2 combined with relay lens systems. In this optical system, an image of an object O.sub.1 formed by the objective lens system is transmitted sequentially as O.sub.2, O.sub.3 and O.sub.4 through relay lens systems R.sub.1, R.sub.2, R.sub.3, . . . , and position of the pupil determining brightness is also transmitted simultaneously. Though S corresponds to the pupil position in the objective lens system, S.sub.1, S.sub.2 and S.sub.3 correspond to pupil positions in the relay lens systems, and diameter of the relay lens systems is equal to that of the pupils S.sub.1, S.sub.2 and S.sub.3 in most cases. For this reason, brightness of optical system is determined almost by outside diameter of the relay lens systems and it is not always necessary to arrange an aperture stops having the light-shielding effect at the pupil position S of the objective lens system.
When inclination angle relative to the optical axis .theta. of the principal ray P incident on the lens unit L.sub.1 (L.sub.1 ') located on the object side of the aperture stop is compared with inclination angle relative to the optical axis .theta.' of the principal ray P emerging from the lens unit L.sub.1 (L.sub.1 ') and incident on the lens unit L.sub.2 (L.sub.2 ') arranged on the image side of the aperture stop in the objective lens systems shown in FIG. 1 and FIG. 2, .theta.' is far smaller than .theta.. This is because the lens unit L.sub.1 has negative refractive function for widening the visual field angle.
In such objective lens systems, there is established the following relationship between the small angle .theta.' and aberrations. Speaking concretely, curvature of field, astigmatism and distortion are produced in small quantities in Seidel's aberrations, whereas spherical aberration and coma are produced in relatively large quantities. This relationship is visualized in FIG. 6. Accordingly, it is sufficient to design the lens unit L.sub.2 having positive refractive power in such a manner that it forms an image having spherical aberration and coma corrected on an assumption that the pupil S located between the lens unit L.sub.1 and the lens unit L.sub.2 is an object. The sine condition is known as the requirement for designing a lens unit for forming such an image. When image height is represented by I, focal length of the lens unit L.sub.2 is designated by f.sub.2 and the inclination angle relative to the optical axis of the principal ray P is denoted .theta.', the sine condition for the telecentric optical system wherein the principal ray P is incident perpendicularly on the image surface I can be expressed by the following formula: EQU I=f.sub.2 sin .theta.'
Further, for the lens unit L.sub.1, the sine condition is satisfied to a certain degree also on the front side of the aperture stop when the lens unit L.sub.1 is composed of a single spherical lens element as shown in FIG. 7. Accordingly we obtain the following relationship: EQU I=f sin .theta.
wherein the reference symbol f represents focal length of the objective lens system as a whole and the reference symbol .theta. designates inclination angle relative to the optical axis of the principal ray P incident on the lens unit L.sub.1.
Almost all the objective lens system for endoscopes which are used currently satisfy the abovementioned sine condition since the lens systems have small outside diameters and comprise small numbers of lens elements.
When the above-mentioned sine condition is satisfied, distortion is aggravated abruptly as field angle .theta. is enlarged as shown in FIG. 6. Relationship between distortion and field angle .theta. can be expressed by the following formula: EQU DT(.theta.)=(cos .theta.-1).times.100 (%)
wherein the reference symbol DT represents distortion which is given by the following formula: ##EQU2## wherein the reference symbol y represents size of an image deformed by distortion and the reference symbol y.sub.0 designates size of an ideal image calculated by the paraxial theory.
When the sine condition is satisfied and distortion has the above-mentioned relationship with the inclination angle .theta., negative distortion (barrel-shaped distortion) is aggravated abruptly as the inclination angle .theta. is enlarged.
The conventional objective lens system for endoscopes shown in FIG. 2, for example, has the following sine condition: EQU I=0.91 EQU f sin .theta.=1.27.times.sin 45.degree.=0.898
Further, the above-mentioned objective lens system for endoscopes has distortion expressed below: EQU DT (.omega..sub.1 =45.degree.)=-30% EQU (cos 45.degree.-1).times.100 =-29.3%
Further, in an objective lens system almost satisfying the sine condition of I=f sin .theta. (hereinafter referred to as an objective lens system of I=f sin .theta. type), DT(.theta.) has the values listed below at various values of .theta.:
______________________________________ Field angle (2.theta.) 40.degree. 60.degree. 80.degree. 100.degree. 120.degree. 140.degree. Distortion (DT(.theta.)) -6 -13.5 -23 -36 -50 -66 (%) ______________________________________
FIG. 8 and FIG. 9 exemplify how images affected by distortion actually look like. FIG. 9 visualized images of the lattice patterns arranged vertically and horizontally at equal intervals on a plane perpendicular to the optical axis shown in FIG. 8 which are formed by using objective lens systems having DT's of 0% to 30% at the maximum image height.
As is understood from the foregoing description, the conventional objective lens systems have remarkable negative distortion though the lens systems have wide field angles which are indispensable for these objective lens systems for endoscopes, are designed as telecentric lens systems, and satisfy the requirements of favorably corrected aberrations and compact design.
The objective lens systems for endoscopes having distortion form images of marginal portions of an object which are smaller and more distorted than the images of central portions. Accordingly, it is impossible to measure or analyze shapes accurately by applying the objective lens system having distortion to inspection of industrial products, or erroneous diagnoses may be made by using such objective lens system in the field of medicine.
Further, in a wide-angle photographic lens system having little distortion and a wide angle as shown in FIG. 10, for example, the following relationship establishes: EQU I=f tan .theta.
In an objective lens system satisfying the abovementioned relationship, light quantity reduces on the image surface thereof at a rate of cos.sup.4 .theta. as .theta. has larger values. In the conventional objective lens system for endoscopes having the remarkable negative distortion, on the other hand, images of the marginal portions are smaller than those of the central portions when portions of the same size over the entire range from the center to the marginal areas are observed, and the reduction of the image size cancels the reduction of light quantity at the rate of cos.sup.4 .theta.. Accordingly, in the objective lens systems satisfying the relationship of I=f sin .theta., brightness is not reduced as inclination angle .theta. is enlarged but uniform over the entire range from the center to the marginal areas. In other words, the objective lens systems for endoscopes satisfying the sine condition have the above-described defect due to the distortion though such lens system can provide uniform brightness.
Furthermore, it is possible to compose oblique viewing, side viewing and backward viewing objective lens systems for endoscopes by combining the objective lens systems for endoscopes with the field direction changing prisms shown in FIG. 3A, FIG. 3B and FIG. 3C as already described.
When the field direction changing prisms are arranged at positions slightly deviated, however, these prisms adversely affect images formed by the objective lens systems.
A field direction changing optical system for oblique viewing is exemplified in FIG. 11 wherein a correcting prism 2 is arranged before a field direction changing prism 1 for correcting astigmatism. In addition, the negative lens unit L.sub.1 is arranged on the object side of the correcting prism 2.
In this field direction changing optical system, the center of the light bundle emerging from the exit surface 1a of the prism 1 shown in FIG. 11 is deviated from the optical axis of the lens unit L.sub.2 due to deviation of arrangement location of the prism caused at the stage of assembly and/or manufacturing error of the prism shape. When the center of the emerging light bundle is deviated from the optical axis of the lens unit L.sub.2 as described above, the light l which is to travel in alignment with the optical axis of the lens unit L.sub.2 is incident on the lens unit L.sub.1 at an angle of .theta. relative to the optical axis of the lens unit L.sub.1, thereby degrading image quality. Further, image quality is degraded also when the prism 1 is arranged in a position inclined relative to the lens unit L.sub.2.